1. Field of the Invention
The present invention is generally related to a method of patterning a preimidized benzophenone photoactive polymer used as a photoresist and, more particularly, to a method of using an excimer laser to improve the crosslinking efficiency of the to polymer.
2. Description of the Prior Art
Advances in semiconductor devices have necessitated the use of greater wiring density in electronic packages and more efficient processing methods. One method of reducing manufacturing time involves using photosensitive polyimide precursors as photoresists in place of standard polyimides, since the procedure employed for the photosensitive layers eliminates a number of steps normally required when conventional polyimide layers are employed. However, most photosensitive polyimides cannot withstand the high temperatures required during semiconductor and substrate manufacturing without flowing, which results in shape alteration, change in line widths and the like. Consequently, resist films comprising photosensitive polyimides are typically treated with a deep ultraviolet (DUV) light source to crosslink and harden the film. The DUV hardening process forms a thin highly crosslinked hard shell at the surface of the resist film at a wavelength of less than 290 nm. The resist film must then be further heated beyond the flow point to high enough temperatures to initiate widespread thermal crosslinking in the bulk of the resist. A photosensitive polyimide which is commonly used in semiconductor and/or substrate processing is a preimidized benzophenone photoactive polymer, commercially available as Ciba-Geigy 412. In a preferred processing method, an 7-10 .mu.m thick film of the polymer is spin applied to a substrate and baked at 95.degree.-105.degree. C. A pattern is then projection exposed on the polymer using a mercury arc lamp at an output of 1200 mJ/cm2 at 365 nm to initiate crosslinking. Exposure at 365 nm will typically crosslink the polymer to the extent that molecular weight is doubled or quadrupled in the exposed areas of the film and, therefore, becomes insoluble in the cosolvent developer. The unexposed portion of the polymer remains chemically unchanged and solvent in the cosolvent developer. The pattern is then developed in a 50/50 cosolvent of xylene and .GAMMA.-butyralactone to reveal the pattern. A major drawback of patterning using a mercury arc lamp is that during development, the crosslinked portion of the polymer swells up to 100% of its original thickness, thereby causing the sidewalls of two adjacent lines of polymer to swell enough that they touch. This is due to the fact that only 3% or less of the polymer crosslinks. When the polymer is baked to remove the solvent, the polymer shrinks the swollen sidewalls which then separate to form strings of polymer which cross from one sidewall to the other across the open pattern. These strings of polymer interfere with the metallization of the pattern. Thus, patterning of the 412 polymer has been limited to the formation of vias on the order of millimeters in size.
U.S. Pat. No. 4,826,756 discloses a process for fully hardening a Novolak resin at a low temperature. The method utilizes a laser which provides a high power output at 308 nm to crosslink the resin. A hotplate, which is held at a constant temperature of about 120.degree. C. is used to accelerate the crosslinking reaction.